The present invention relates to a power-conversion apparatus comprising a bridge circuit, each arm of which has multiple semiconductor devices connected in parallel, and in particular to a power-conversion apparatus that improves the balance of the current share of each semiconductor device.
FIG. 6 shows a connection diagram of a main circuit of a rectifier used for an electrolytic cell facility as a power-conversion apparatus.
In FIG. 6, 1 indicates a bus for an AC-side first phase, 2 indicates a bus for an AC-side second phase, 3 indicates a bus for an AC-side third phase, 4 indicates a bus for a DC-side positive pole, 5 indicates a bus for a DC-side negative pole, 11 to 34 indicate Thyristers used as semiconductor devices, and 41 to 64 indicate connection conductors extending from one of the buses 1 to 3 to the Thyristers 11 to 34. In a main circuit of this rectifier, the Thyristers 11 to 14 are connected in parallel so as to form a U-phase arm, the Thyristers 15 to 18 are connected in parallel so as to form a V-phase arm, the Thyristers 19 to 22 are connected in parallel so as to form a W-phase arm, the Thyristers 23 to 26 are connected in parallel so as to form a X-phase arm, the Thyristers 27 to 30 are connected in parallel so as to form a Y-phase arm, and the Thyristers 31 to 34 are connected in parallel so as to form a Z-phase arm. The entire apparatus is composed as a three-phase bridge circuit by arranging the U- to W-phase arms as, for example, an upper arm, while arranging the X- to Z-phase arms as, for example, a lower arm.
FIGS. 7(a) and 7(b) are typical conceptual diagrams showing a conventional example of the main circuit of the rectifier shown in FIG. 6, wherein the Thyristers 11 to 34 have flat structures.
In the configuration shown in FIGS. 7(a) and 7(b), cathodes of the Thyristers 11 to 22 of each of the U- to W-phase arms contact one surface of the bus 4 for the DC-side positive pole, and anodes of the Thyristers 11 to 22 are connected to one of buses 1 to 3 (not shown) via the connection conductors 41 to 52. In addition, anodes of the Thyristers 23 to 34 of each of the X- to Z-phase arms contact one surface of a bus 5 for the DC-side negative pole that forms the back of the surface positioned opposite to the bus 4 for the DC-side positive pole, and cathodes of the Thyristers 23 to 34 are connected to one of buses 1 to 3 (not shown) via the connection conductors 53 to 64. As shown in FIG. 7(a), the Thyristers 11 to 22 are linearly arranged on one surface of the bus 4, while the Thyristers 23 to 34 are linearly arranged on one surface of the bus 5.
In the configuration shown in FIG. 7(a), when a current begins to flow through, for example, the U-phase arm, a voltage drop occurs across the connection conductors 41 to 44, as shown in Equations (1) to (4).
In each of Equations (1) to (12), the subscripts indicate the connection conductors 41 to 64 and the intermediate positions between the conductors.
Equation 1 EQU V.sub.41 =R.sub.41 .multidot.i.sub.41 +L.sub.41 .multidot.di.sub.41 /dt+M.sub.41-42 .multidot.di.sub.42 /dt+M.sub.41-43 .multidot.di.sub.43 /dt+M.sub.41-44 .multidot.di.sub.44 /dt (1)
Equation 2 EQU V.sub.42 =R.sub.42 .multidot.i.sub.42 +L.sub.42 .multidot.di.sub.42 /dt+M.sub.41-42 .multidot.di.sub.41 /dt+M.sub.42-43 .multidot.di.sub.43 /dt+M.sub.42-44 .multidot.di.sub.44 /dt (2)
Equation 3 EQU V.sub.43 =R.sub.43 .multidot.i.sub.43 +L.sub.43 .multidot.di.sub.43 /dt+M.sub.41-43 .multidot.di.sub.41 /dt+M.sub.42-43 .multidot.di.sub.42 /dt+M.sub.43-44 .multidot.di.sub.44 /dt (3)
Equation 4 EQU V.sub.44 =R.sub.44 .multidot.i.sub.44 +L.sub.44 .multidot.di.sub.44 /dt+M.sub.41-44 .multidot.di.sub.41 /dt+M.sub.42-44 .multidot.di.sub.42 /dt+M.sub.43-44 .multidot.di.sub.43 /dt (4)
In Equations (1) to (4), V indicates voltage, R indicates resistance, (i) indicates current, L indicates self-inductance, M indicates mutual inductance, and di/dt indicates a degree of temporal change in the current (i).
In Equations (1) to (4), if the variations in the forward voltage drops of the Thyristers 11 to 14 of the U-phase arm and the variations in the characteristic constants of the connection conductors 41 to 44 are ignored, the voltage drops V41 to V44 are equal across the connection conductors 41 to 44.
In the configuration shown in FIG. 7(a), however, the mutual inductances M between the connection conductors 41 to 44 are inversely proportional to the distance between the conductors. Thus, in Equations (1) to (4), currents i.sub.41, i.sub.44 flowing through the connection conductors 41, 44 are larger than currents i.sub.42, i.sub.43 flowing through the connection conductors 42, 43. Consequently, currents flowing through the Thyristers 11, 14 are larger than currents flowing through the Thyristers 12, 13. These differences occur while each current is temporally varying (di/dt.noteq.0).
In addition, in the configuration shown in FIG. 7(a), while a current is flowing through, for example, the Y-phase arm, if a current begins to flow through the U-phase arm in the opposite direction relative to the Y-phase arm, the voltage drops shown in Equations (5) to (8) occur across the connection conductors 41 to 44.
Equation 5 EQU V.sub.41 =R.sub.41 .multidot.i.sub.41 +L.sub.41 .multidot.di.sub.41 /dt-M.sub.41-57 .multidot.di.sub.57 /dt-M.sub.41-58 .multidot.di.sub.58 /dt-M.sub.41-59 .multidot.di.sub.59 /dt-M.sub.41-60 .multidot.di.sub.60 /dt(5)
Equation 6 EQU V.sub.42 =R.sub.42 .multidot.i.sub.42 +L.sub.42 .multidot.di.sub.42 /dt-M.sub.42-57 .multidot.di.sub.57 /dt-M.sub.42-58 .multidot.di.sub.58 /dt-M.sub.42-59 .multidot.di.sub.59 /dt-M.sub.42-60 .multidot.di.sub.60 /dt(6)
Equation 7 EQU V.sub.43 =R.sub.43 .multidot.i.sub.43 +L.sub.43 .multidot.di.sub.43 /dt-M.sub.43-57 .multidot.di.sub.57 /dt-M.sub.43-58 .multidot.di.sub.58 /dt-M.sub.43-59 .multidot.di.sub.59 /dt-M.sub.43-60 .multidot.di.sub.60 /dt(7)
Equation 8 EQU V.sub.44 =R.sub.44 .multidot.i.sub.44 +L.sub.44 .multidot.di.sub.44 /dt-M.sub.44-57 .multidot.di.sub.57 /dt-M.sub.44-58 .multidot.di.sub.58 /dt-M.sub.44-59 .multidot.di.sub.59 /dt-M.sub.44-60 .multidot.di.sub.60 /dt(8)
In Equations (5) to (8), V indicates voltage, R indicates resistance, (i) indicates current, L indicates self-inductance, M indicates mutual inductance, and di/dt indicates a degree of temporal change in current (i).
In Equations (5) to (8), if the variations in the edgewise voltages in the Thyristers 11 to 14 of the U-phase arm and the variations in the characteristic constants of the connection conductors 41 to 44 are ignored, the voltage drops V41 to V44 are equal across the connection conductors 41 to 44.
In the configuration shown in FIG. 7(a), however, the mutual inductance M between each of the connection conductors 41 to 44 and each of the connection conductors 57 to 60 is inversely proportional to the distance between the conductors. Thus, in Equations (5) to (8), current i.sub.44 flowing through the connection conductor 44 is larger than currents i.sub.41, i.sub.42, i.sub.43 flowing through the connection conductors 41 to 43. Consequently, the current flowing through the Thyrister 14 is larger than the currents flowing through the Thyristers 11 to 13. These differences occur while each current is temporally varying (di/dt.noteq.0).
In addition, in the configuration shown in FIG. 7(a), while a current is flowing through, for example, the U-phase arm, and the current through the Y-phase arm is commuted to the current through the Z-phase arm, the voltage drops shown in Equations (9) to (12) occur across the connection conductors 41 to 44.
Equation 9 EQU V.sub.41 =R.sub.41 .multidot.i.sub.41 +L.sub.41 .multidot.di.sub.41 /dt+M.sub.41-57 .multidot.di.sub.57 /dt+M.sub.41-58 .multidot.di.sub.58 /dt+M.sub.41-59 .multidot.di.sub.59 /dt+M.sub.41-60 .multidot.di.sub.60 /dt-M.sub.41-61 .multidot.di.sub.61 /dt-M.sub.41-62 .multidot.di.sub.62 /dt-M.sub.41-63 .multidot.di.sub.63 /dt-M.sub.41-64 .multidot.di.sub.64 /dt(9)
Equation 10 EQU V.sub.42 =R.sub.42 .multidot.i.sub.42 +L.sub.42 .multidot.di.sub.42 /dt+ EQU M.sub.42-57 .multidot.di.sub.57 /dt+M.sub.42-58 .multidot.di.sub.58 /dt+ EQU M.sub.42-59 .multidot.di.sub.59 /dt+M.sub.42-60 .multidot.di.sub.60 /dt- EQU M.sub.42-61 .multidot.di.sub.61 /dt-M.sub.42-62 .multidot.di.sub.62 /dt- EQU M.sub.42-63 .multidot.di.sub.63 /dt-M.sub.42-64 .multidot.di.sub.64 /dt(10)
Equation 11 EQU V.sub.43 =R.sub.43 .multidot.i.sub.43 +L.sub.43 .multidot.di.sub.43 /dt+M.sub.43-57 .multidot.di.sub.57 /dt+M.sub.43-58 .multidot.di.sub.58 /dt+M.sub.43-59 .multidot.di.sub.59 /dt+M.sub.43-60 .multidot.di.sub.60 /dt-M.sub.43-61 .multidot.di.sub.61 /dt-M.sub.43-62 .multidot.di.sub.62 /dt-M.sub.43-63 .multidot.di.sub.63 /dt-M.sub.43-64 .multidot.di.sub.64 /dt(11)
Equation 12 EQU V.sub.44 =R.sub.44 .multidot.i.sub.44 +L.sub.44 .multidot.di.sub.44 /dt+M.sub.44-57 .multidot.di.sub.57 /dt+M.sub.44-58 .multidot.di.sub.58 /dt+M.sub.44-59 .multidot.di.sub.59 /dt+M.sub.44-60 .multidot.di.sub.60 /dt-M.sub.44-61 .multidot.di.sub.61 /dt-M.sub.44-62 .multidot.di.sub.62 /dt-M.sub.44-63 .multidot.di.sub.63 /dt-M.sub.44-64 .multidot.di.sub.64 /dt(12)
In Equations (9) to (12), V indicates voltage, R indicates resistance, (i) indicates current, L indicates self-inductance, M indicates mutual inductance, and di/dt indicates a degree of temporal change in current (i).
In Equations (9) to (12), if the variations in the edgewise voltages in the Thyristers 11 to 14 of the U-phase arm and the variations in the characteristic constants of the connection conductors 41 to 44 are ignored, the voltage drop V.sub.41 to V.sub.44 is equal across the connection conductors 41 to 44.
In the configuration shown in FIG. 7(a), however, the mutual inductance M between each of the connection conductors 41 to 44 and each of the connection conductors 57 to 64 is inversely proportional to the distance between the conductors. Thus, in Equations (9) to (12), current i.sub.44 flowing through the connection conductor 44 is larger than currents i.sub.41, i.sub.42, i.sub.43 flowing through the connection conductors 41 to 43. Consequently, the current flowing through the Thyrister 14 is larger than the currents flowing through the Thyristers 11 to 13. These differences occur while each current is temporally varying (di/dt.noteq.0).
In the conventional configuration of a main circuit of a rectifier used as a power-conversion apparatus, a current unbalance occurs among the multiple semiconductor devices constituting each arm.
Therefore, to eliminate this current unbalance, the number of semiconductor devices connected in parallel is increased, resulting in larger and more expensive power-conversion apparatuses.
In addition, as is well known, the magnitude of the edgewise voltage has been individually determined for each semiconductor device of a power-conversion apparatus in order to reduce current unbalance. Recent advancements in semiconductor fabrication technologies, however, have contributed to a reduction in variation in the edgewise voltages of the semiconductor devices. As a result, a large amount of time and labor are required for this operation.
It is an object of this invention to provide a power-conversion apparatus that solves these problems.